Downloaded from http://cshperspectives.cshlp.org/ on October 1, 2021 - Published by Cold Spring Harbor Laboratory Press Mechanisms of Small Heat Shock Proteins Maria K. Janowska, Hannah E.R. Baughman, Christopher N. Woods, and Rachel E. Klevit Department of Biochemistry, University of Washington, Seattle, Washington 98195 Correspondence: [email protected] Small heat shock proteins (sHSPs) are ATP-independent chaperones that delay formation of harmful protein aggregates. sHSPs’ role in protein homeostasis has been appreciated for decades, but their mechanisms of action remain poorly understood. This gap in understand- ing is largely a consequence of sHSP properties that make them recalcitrant to detailed study. Multiple stress-associated conditions including pH acidosis, oxidation, and unusual avail- ability of metal ions, as well as reversible stress-induced phosphorylation can modulate sHSP chaperone activity. Investigations of sHSPs reveal that sHSPs can engage in transient or long- lived interactions with client proteins depending on solution conditions and sHSP or client identity. Recent advances in the field highlight both the diversity of function within the sHSP family and the exquisite sensitivity of individual sHSPs to cellular and experimental condi- tions. Here, we will present and highlight current understanding, recent progress, and future challenges. lthough small heat shock proteins (sHSPs) et al. 2012). sHSPs are implicated in muscle pro- Awere recognized as protein chaperones a tection, their expression is associated with poor quarter century ago (Horwitz 1992; Jakob et al. prognosis and treatment resistance in cancer, 1993), understanding how they work at a molec- and they play ameliorative roles in Parkinson’s ular level has been slow to emerge. sHSPs are and Alzheimer’s disease (Zoubeidi and Gleave defined by their shared α-crystallin domain 2012; Dubińska-Magiera et al. 2014; Leak (ACD), named after the highly abundant sHSPs 2014). Transcription of some, but not all, sHSPs in the eye lens, αA-crystallin (referred to here is under the control of the heat shock fac- by its gene name, HSBP4) and αB-crystallin tor (HSF) transcription factors, which can up- (HSPB5) (Caspers et al. 1995). sHSPs are ATP- regulate the cellular concentrations of an sHSP independent chaperones that can delay the onset in response to stress (Table 1) (De Thonel et al. of irreversible protein aggregation in response to 2012; Zhong et al. 2016). In addition, sHSPs cellular stressors. Mutations in sHSPs are linked themselves are exquisitely sensitive to their con- to multiple diseases, including various neurop- ditions, and their activity, as well as their protein athies and early-onset cataract formation, im- levels, is activated by cellular conditions (Hasl- plying that sHSP dysfunction can have dire con- beck et al. 2005; Treweek et al. 2015). Genomes sequences (Litt et al. 1998; Vicart et al. 1998; across biology contain varying numbers of Irobi et al. 2004; Kijima et al. 2005; Hansen sHSPs: Escherichia coli and Saccharomyces cer- et al. 2007; Houlden et al. 2008; Datskevich evisiae each have two; Drosophila melanogaster Editors: Richard I. Morimoto, F. Ulrich Hartl, and Jeffery W. Kelly Additional Perspectives on Protein Homeostasis available at www.cshperspectives.org Copyright © 2019 Cold Spring Harbor Laboratory Press; all rights reserved Advanced Online Article. Cite this article as Cold Spring Harb Perspect Biol doi: 10.1101/cshperspect.a034025 1 Downloaded from http://cshperspectives.cshlp.org/ on October 1, 2021 - Published by Cold Spring Harbor Laboratory Press M.K. Janowska et al. Table 1. Basic information regarding human small heat shock proteins (sHSPs) Gene name Other names Tissue distribution Heat shock factor (HSF) inducible HSPB1 Hsp25, Hsp27, Hsp28 Ubiquitous HSF-1, HSF-2 HSPB2 MKBP Cardiac and skeletal muscle HSPB3 HSPL27 Cardiac and skeletal muscle HSPB4 αA-crystallin Eye lens HSPB5 αB-crystallin Ubiquitous HSF-1 HSPB6 Hsp20, p20 Ubiquitous HSPB7 cvHsp Cardiac and skeletal muscle HSPB8 Hsp22 Ubiquitous HSF-1 HSPB9 CT51 Testis HSPB10 ODF1 Testis has 12; Caenorhabditis elegans has 16; and terminal region (NTR) and a flexible carboxy- Arabidopsis thaliana has 25 (Susek and Lind- terminal region (CTR) flank the structured ACD quist 1989; Laskowska et al. 1996; Wotton et al. (Fig. 1). The three domains show distinct be- 1996; Scharf et al. 2001; Candido 2002; Michaud haviors that arise, at least in part, from their et al. 2002). The ten human sHSPs differ in their distinct amino acid content. The ACD is the tissue distribution and response to specific only natively folded region, forming an IgG- stressors (see Table 1) (Kappé et al. 2003). like β-sandwich structure (Fig. 1B). ACDs of hu- Recent studies have begun to define how man sHSPs are enriched in histidines that may sHSPs become activated and how they recognize give rise to an ability to respond to changes in “clients” (the proteins on which they act) (Mainz pH and in metal ion availability to modulate et al. 2012; Peschek et al. 2013; Rajagopal et al. sHSP activity (Fig. 1A). The CTR is enriched 2015b). Such studies are proving to be highly in polar and charged residues, is highly disor- informative, although their interpretation in dered, and is thought to serve as a solubility tag terms of general models for sHSP activity has to enable the extremely high concentrations of proven challenging. As sHSPs act as early re- sHSP found in tissues such as eye lens (>150 mg/ sponders to help cells cope with proteins that mL) to remain soluble (Smulders et al. 1996; are destabilized because of a stress condition, Horwitz 2003). NTRs are enriched in hydropho- the “client-ome” could be vast, diverse, and dif- bic residues and are disordered (Bloemendal ferent depending on the state of the cell before 1977). Despite their simple architecture, sHSPs the stress. The sheer diversity of potential clients are structurally complicated. Human sHSPs ex- makes it unclear whether results obtained on ist in a range of oligomeric states. Some, includ- specific systems can be generalized to other ing HSPB1, HSPB4, and HSPB5, form polydis- sHSP systems and whether it is sensible to try perse ensembles of oligomers that range in size to do so. Here, we discuss emerging models and from dimers to ∼40-mers (Aquilina et al. 2003; outstanding questions regarding sHSP mecha- Horwitz 2003; Jovcevski et al. 2015). These olig- nisms and suggest strategies to leverage both omeric ensembles are highly dynamic with fre- technological and scientific developments to im- quent subunit exchange between oligomers (Pe- prove understanding of these enigmatic but crit- schek et al. 2013). Other sHSPs, such as HSPB8 ical proteins. and HSPB6 exist predominantly as small oligo- mers or dimers (Bukach et al. 2004). To date, there is no evidence of an sHSP that exists pre- sHSP STRUCTURE dominantly as a monomer, but such species may exist fleetingly as subunits exchange from one sHSPs Have Unusual Structural Properties oligomer to another (Bova et al. 1997). Like all sHSPs, the human sHSPs share a domain Although oligomeric mammalian sHSPs are architecture in which a highly variable amino- recalcitrant to conventional structural biology 2 Advanced Online Article. Cite this article as Cold Spring Harb Perspect Biol doi: 10.1101/cshperspect.a034025 Downloaded from http://cshperspectives.cshlp.org/ on October 1, 2021 - Published by Cold Spring Harbor Laboratory Press Mechanisms of Small Heat Shock Proteins A HSPB1 1 ----MTERRVP-FS-LLRGPSWDPFRDWYPHSRLFDQAFGLPRLPEEWSQWLG-------GSSWPG--------------YVRPLPPA 61 HSPB2 1 ----MSGRSVP-HA----HPAT-AEYEFANPSRLGEQRFGEGLLPEEIL--TP-------TLYHGY--------------YVRP---- 51 HSPB3 1 ----MAKIILR-HL-------------IEIPVRYQEEFEARGLEDCRLDHALY-------ALPGPT--------------IVDL---- 45 HSPB4 1 -----MDVTIQ-HP-WFKRTLG-P---FY-PSRLFDQFFGEGLFEYDLLPFLS-------STISPY--------------YRQ----- 50 HSPB5 1 -----MDIAIH-HP-WIRRPFF-P---FHSPSRLFDQFFGEHLLESDLFP-TS-------TSLSPF--------------YLRP---- 51 HSPB6 1 -----MEIPVPVQPSWLRRASA-PLPGLSAPGRLFDQRFGEGLLEAELAALCP-------TTLAPY--------------YLRA---- 57 HSPB7 1 -----MSHRTS-STFRAERSFH-SSSSSSSSSTSSSASRALPAQDPPMEKALSMF-----SDDFGS--------------FMRP---- 58 HSPB8 1 MADGQMPFSCH-YPSRLRRD---PFRDSPLSSRLLDDGFGMDPFPDDLTASWPDWALPRLSSAWPG--------------TLRS---- 66 HSPB9 1 ------MQRVG-NT-------------FSNESRVASRCPSVGLAERNRVATMP----------------------------VRLL--- 37 HSPB10 1 ----MAALSCL-LD-SVRRDIKKVDREL-RQLRCIDEFSTRCLCDLYMHPYCC-------CDLHPYPYCLCYSKRSRSCGLCDLYPCC 74 Sip1 1 -----MSSLCP-YT--------------GRPTGLFRDF----------------------EDMMPY--------------WAQR---- 28 HSPB1 62 AIESPAVAAPAYSRALSRQLSSGV-----SEIRHTADRWRVS------------LDVNHFAPDELTVKTKDGVVEITGKHEERQDEHG 132 HSPB2 52 ---------RAAPAGEG--SRAGA-----SELRLSEGKFQAF------------LDVSHFTPDEVTVRTVDNLLEVSARHPQRLDRHG 111 HSPB3 46 ---------RKTRAAQSPPVDSAAE----TPPREGKSHFQIL------------LDVVQFLPEDIIIQTFEGWLLIKAQHGTRMDEHG 108 HSPB4 51 ----------SLFRTV---LDSGI-----SEVRSDRDKFVIF------------LDVKHFSPEDLTVKVQDDFVEIHGKHNERQDDHG 108 HSPB5 52 ---------PSFLRAPSW-FDTGL-----SEMRLEKDRFSVN------------LDVKHFSPEELKVKVLGDVIEVHGKHEERQDEHG 112 HSPB6 58 ---------PSVALPV-------------AQVPTDPGHFSVL------------LDVKHFSPEEIAVKVVGEHVEVHARHEERPDEHG 111 HSPB7 59 ---------HSEPLAFPA-RPGGA-----GNIKTLGDAYEFA------------VDVRDFSPEDIIVTTSNNHIEVRA---EKLAADG 116 HSPB8 67 ---------GMVPRGPTATARFGVPAEGRTPPPFPGEPWKVC------------VNVHSFKPEELMVKTKDGYVEVSGKHEEKQQEGG 134 HSPB9 38 ---------RDSPAAQ-------------EDNDHARDGFQMK------------LDAHGFAPEELVVQVDGQWLMVTGQQQLDVRDPE 91 HSPB10 75 LCDYKLYCLRPSLRSLERKAIRAIEDEKRELAKLRRTTNRILASSCCSSNILGSVNVCGFEPDQVKVRVKDGKVCVSAERENRYDCLG